Hyperoxia exposure is an important factor involved in the pathogenesis of bronchopulmonary dysplasia (BPD). BPD is a major long-term morbidity in premature infants, which imposes long-term medical and societal burden for the affected infants. Impaired lung vascular and alveolar growth is a characteristic feature of BPD histology. Recent studies demonstrated that poor blood vessel growth in the lungs may be the critical factor involved in the development of BPD. Blood vessel growth is controlled by several angiogenic growth factors and nitric oxide (NO) released by endothelial NO synthase (eNOS), appears to be the key mediator for angiogenic growth factors. Adequate eNOS function relies on several co-factors, including tetrahydrobiopterin (BH4). BH4 is formed by either de novo pathway or by salvage pathway. GTP-chclohydrolase-1 (GCH1) is the rate-limiting enzyme of intracellular BH4 synthesis. Decreased intracellular BH4 levels, or oxidation of BH4 to BH2, uncouple eNOS activity with more reactive oxygen species (ROS) forming in the endothelial cells instead of NO. Decreased NO formation can increase pulmonary artery vasomotor tone and aggravate respiratory distress in premature infants. Premature infants with respiratory distress often require the use of oxygen and mechanical ventilation; both of which increase ROS formation in premature lungs. Pulmonary artery endothelial cells from premature lambs have decreased BH4 levels and GCH1 expression, and eNOS uncoupling after exposure to hyperoxia, whereas, sepiapterin partially corrects these changes. We hypothesize that increasing endogenous BH4 in the lung can protect premature lungs from hyperoxic injury. We plan to investigate our hypothesis in Sprague-Dawley rat pups exposed to hyperoxia during the first 10 days of postnatal life. Studies under specific aim 1 will investigate BH4 levels and the expression of BH4 forming enzymes in their lungs during hyperoxia. Expression of GCH1 will be studied at transcriptional, translational, and post-translational levels by chromatin immunoprecipitation, DNA methylation, quantitative real-time polymerase chain reaction, and immunoblotting. Studies under specific aim 2 will investigate the mechanistic link between BH4 depletion and hyperoxia-induced impairment of lung growth. These studies will be done by supplementing rat pups with sepiapterin during hyperoxia exposure, to increase BH4 levels. Lung histology and pulmonary artery reactivity will be used to investigate the protective effect of increasing endogenous BH4 levels. This pilot study will provide information on the potential use of sepiapterin, a BH4 analogue, in preventing or treating BPD.